It won't delete them directly. If you set it low enough to make them not friends, they'll disappear from the "friends" tab, but they'll still be on the "everyone" tab. But if you set the relationship to 0 and then don't interact with them for a few days, they do seem to disappear.
There might be a more direct way with Deaderpool's MCCC mod, but I don't use that mod enough to know. Someone else might have an answer for that if you go over to the mod request thread:
https://forums.thesims.com/en_US/discussion/867567/mod-request-thread/p1